Cheers for your help AceJavelin, 
We have got the systems sharing data and get no SDS errors, we do however get this error when looking into the Maintenance Logs.
Log Type      Software
Log Number      2046
Severity      Warning
Date      2016/Apr/20
Time      17:39:48
Source      Call Control - Software
Description      
SWERR range_check_error, OFFSET 2:$15EC, PC: $5D182A LOG_ID: xnetlh#98, PROCEDURE: xnetlh_handle_rmsg_open_ind, SEVERITY: cp_inform PID: ($2, $60C) Xnet Rejected open for ICP: 001 Received Invalid IP: 192.168.121.254
Module      Main
File Name and Line Number      log_outputs_util_en.c;1227
I am now suspecting the routers between sites to be modifying the packets in some way but I will confirm with the IT company tomorrow. As VisualSpirit says, there are no firewalls, just routers on an MPLS.